12-speed cassettes would be overkill for me, no question. Frankly I have no yearning for 11-speed either.

It's the gaps, not so much the range. Evidently there are enough cyclists who have a narrow range of comfortable cadence that smaller jumps are attractive - no conspiracy, not marketing malarkey, just simply demand. Those of us with a wider range don't see the point.

I do question whether the ever-smaller gaps will remain attractive, but I also see the possibility. Imagine a 15 cog cassette (assuming it could be reliable and durable). I think it would feel like a continuously variable transmission - indexing might become irrelevant and you'd just dial it in. Why not, variable is attractive in most ways except for transmission efficiency and using a huge number of gears would fix that.